Depression is the most common mental illness, but there's still a lot we don't know about how the condition affects the brain. But now, a new MRI study reveals that depression deactivates vital pieces of brain circuitry... including, surprisingly, the region that controls hatred.

Researchers at the UK's University of Warwick performed MRI scans on 39 depressed people and 37 control subjects who didn't have the condition. They were able to pinpoint several key differences in the brain circuitry between the two groups. Most of these centered on the uncoupling of connections between various parts of the brain. Normally, these connections form circuits that allow the different areas to work together to produce more complex mental processes. With the circuits uncoupled, the brain should have a harder time performing the related mental tasks.

That's why it's so strange that the so-called "hate circuit" - a circuit connecting the superior frontal gyrus, insula, and putamen that was shown in 2008 to be strongly associated with feelings of hatred - was often uncoupled in the depressed subjects. The researchers discovered this by showing the subjects pictures of people they hated. While the non-depressed people showed clear activity in this hate circuit, most of the depressed people showed no such response. In fact, the hate circuit was 92% more likely to be decoupled in the depressed subjects.

There were similarly high odds of deactivation for the risk and action circuit (92%) and the emotion and reward circuit (82%), but neither of those is really surprising in terms of the general behavior patterns of people dealing with depression, which is often associated with an unwillingness to take risks and difficulty feeling positive emotions. But since depression is known to be associated with self-loathing - which is very much a form of hatred - then how could that fit with the hate circuit being deactivated?

Researcher Jianfeng Feng has a theory:

"The results are clear but at first sight are puzzling as we know that depression is often characterized by intense self loathing and there is no obvious indication that depressives are less prone to hate others. One possibility is that the uncoupling of this hate circuit could be associated with impaired ability to control and learn from social or other situations which provoke feelings of hate towards self or others. This in turn could lead to an inability to deal appropriately with feelings of hate and an increased likelihood of both uncontrolled self-loathing and withdrawal from social interactions. It may be that this is a neurological indication that is more normal to have occasion to hate others rather than hate ourselves."